In dogs and cats food allergies tend to be to proteins. There needs to be prior exposure to the protein to be able to develop an allergy to it. Allergies to preservatives are seen as a totally different scenario in dogs especially and present in a manner much more closely to those signs seen with drug allergies.
Food allergies cannot reliably be tested for with blood tests. The best method is via a period of feeding a diet which has been determined to be hypoallergenic (something the pet hasn